The coating of adhesive polymers, including pressure-sensitive adhesives, such as rubber-based materials, onto suitable backings is well-known in the art. Typically, these adhesive polymers are formed by solution or emulsion polymerization techniques. Thereafter, the resultant adhesive polymers are coated either directly from their reaction medium, or subsequent to isolation and dissolution, onto a substrate. See e.g., D. Satas, Ed., "Handbook of Pressure Sensitive Adhesive Technology," Van Nostrand, New York, Chapter 13, pg. 298 (1982).
Over the past several years, research has focused upon the production of adhesive polymers through solvent-free polymerization techniques. For example, U.S. Pat. Nos. 4,833,179, and 4,952,650 disclose the production of acrylate-based pressure-sensitive adhesive copolymers in an aqueous medium via suspension polymerization techniques. However, to date, these adhesives are isolated from the aqueous medium, dissolved in an organic solvent, and coated therefrom onto a suitable substrate, such as a tape backing material. Thus, while organic solvents can be eliminated as a reaction medium for the polymerization of many adhesive polymers, they are still typically used to coat these adhesive materials.
A number of problems arise from the use of organic solvents as a coating media. Most useful organic solvents are flammable, and thus, can pose a safety hazard, both during and after coating. Also, the relatively high cost of organic solvents can substantially increase the final cost of the adhesive-coated product. Finally, the organic solvents must be removed from the coated adhesive, typically by evaporative drying, after coating. This subsequent removal of organic solvents results in additional energy and equipment costs, and poses safety and environmental disposal problems.
In an effort to overcome the problems associated with organic solvent-coated adhesives, hot-melt extrusion coating of adhesive polymers has been employed. In theory, hot-melt extrusion coating of a dry adhesive polymer from the melt state should provide numerous advantages, including elimination of unwanted organic solvents, and nearly an instantaneous bonding of the hot adhesive to a suitable substrate. However, in practice, useful hot-melt coating can be difficult to achieve, particularly where odor-free adhesive coatings, such as in the medical field, are required. For example, many desirable adhesive polymers, such as acrylate-based pressure-sensitive adhesives, exhibit high viscosities, even at normally elevated temperatures. Accordingly, these adhesive polymers must be subjected to increased processing temperatures to lower their melt viscosity to a sufficient level that a smooth, clear, hot-melt coating can be obtained. However, these higher processing temperatures also lead to significant thermal degradation of the adhesive polymer. It is this thermal degradation that results in unwanted odors, off-color coatings, and potential reduction in adhesive and cohesive properties of the coated adhesive polymer.
Numerous pre-processing and post-processing chemical modifications of adhesive polymers have been proposed to help alleviate this thermal degradation problem. For example, U.S. Pat. No. 4,851,278 discloses thermally-reversible crosslinking, through the use of metal ions, such as zinc, to lower the melt viscosity of acrylate-based adhesives at elevated temperatures. See also, U.S. Pat. Nos. 4,360,638 and 4,423,182. Similarly, U.S. Pat. No. 3,925,282 utilizes both tertiary-amine containing monomers and organometallic salts to lower melt viscosity of acrylate polymers at elevated temperatures. Also, U.S. Patent Nos. 4,554,324, 4,551,388, and 3,558,746 all describe acrylate graft copolymers with favorable viscosity profiles.
Further, U.S. Pat. No. 4,762,888 discloses certain specified mixtures of acrylic copolymers that exhibit thermally-reversible hydrogen bonding. Also, the admixture of photocrosslinking agents to lower-molecular weight polymers, followed by radiation curing after coating, is described in U.S. Pat. Nos. 4,052,527 and 4,234,662. However, in all of these instances, special equipment, unusual and expensive monomers, and/or additives which are undesirable in the final coated adhesive product are required to yield the disclosed results.
In another approach, various processing aids can be added to polymers to limit their thermal degradation during general melt processing. For example, lubricating processing aids, such as fatty acids, fatty alcohols, metallic soaps, waxes, and various inorganic materials, modify the melt flow behavior of polymers, and thereby, limit the degree of thermal degradation to the polymer during melt processing. See e.g., Radian Corporation, "Chemical Additives for the Plastics Industry: Properties, Applications, Toxicologies", pp. 99-101, Noyes Data Corp., Park Ridge, N.J. (1987); "Plastics Additives Handbook: Stabilizers, Processing Aids, Plasticizers, Fillers, Reinforcements, Colorants for Thermoplastics" R. Gachter and H. Muller, Eds., pp 423-467, Hanser, N.Y. (3rd. ed., 1990). However, these processing aids are typically non-volatile materials that remain as a component of the polymer after processing, and thereby, can adversely affect the polymers ultimate properties, such as transparency, toxicity, odor, strength, and adhesive properties. Furthermore, the processing aid may bleed to the surface of the extruded polymer, and impart undesirable surface properties thereto. See "Encyclopedia of Polymer Science and Engineering" H. Mark et al., Eds., Index Volume, pp. 307-324, John Wiley & Sons, New York (1990).
Water is recognized as a processing aid in the melt extrusion of hydrophilic polymers. Specifically, the addition of water to hydrophilic polymers has been shown to lower their melt temperature and melt viscosity during general melt extrusion. For example, U.S. Pat. No. 3,941,865 discloses the addition of water to solid polyethylene oxide resin prior to extrusion, while U.S. Pat. Nos. 4,761,453, 4,876,307, and 4,874,307 disclose injection molding or extrusion of polyketones subsequent to saturation with water. Similarly, U.S. Pat. No. 4,094,948, and B. G. Frushour, 4 Polymer Bulletin, 305-314 (1981); 7 Polymer Bulletin, 1-8 (1982); and 11 Polymer Bulletin, 375-382 (1984), all show that the addition of water to acrylonitrile copolymers reduces their melt temperature for extrusion purposes.
While water has been used as a processing aid for hydrophilic polymers, it is considered to be an incompatible and undesirable additive in the hot-melt extrusion coating of hydrophobic adhesive polymers. For example, it is known that injection of water into a molten hydrophobic polymer can cause the composition to bubble and foam. See e.g., M. Mack, "Choosing an Extruder for Melt Devolatilization" Plastics Engineering, pg. 49 (July, 1986). Thus, it is a fast-held belief by those skilled in the art that inclusion of all but the smallest quantity of water in a hydrophobic adhesive polymer during hot-melt coating will lead to foaming of the polymer extrudate at the coating die/substrate interface, and accordingly, to a defective, nonuseable adhesive coating with bubbles, coating gaps, and other coating irregularities.
Furthermore, the inclusion of water is considered so deleterious to obtaining an effective adhesive coating, that those skilled in the art teach that optimum hot-melt coatings of hydrophobic adhesive polymers can only be obtained by coating as close as possible to a 100% solids, water-free, composition onto a suitable substrate. However, as noted above, this requires the use of costly curing equipment, exotic monomers, or additives with unwanted side-effects to avoid unwanted thermal degradation. Likewise, if the hydrophobic adhesive polymer is coated at a lower melt temperature, the high melt viscosity of the polymer will lead to defects in the adhesive coating, including a whitish translucent coloration, visible melt-flow lines, an irregular "shark skin" surface, as well as other coating irregularities. Thus, present methods of hot-melt coating hydrophobic adhesive polymers necessitate unwanted compromises in coating quality in an effort to hot-melt extrusion coat these materials.